The present invention generally relates to hydrophilic coatings. More specifically, the present invention relates to hydrophilic coatings for heat exchanger surfaces and which coatings can also be anti-microbial and corrosion resistant in nature.
An environmental control system for a manned spacecraft typically includes a condensing heat exchanger for controlling cabin temperature and humidity. Cabin air is drawn into the condensing heat exchanger and circulated through air passageways. Heat drawn from the drawn-in air is transferred to a coolant that is circulated through coolant passageways within the condensing heat exchanger. As the drawn-in air is being cooled, condensate forms on the heat transfer surfaces of the air passageways. Water droplets, if allowed to form, can cause problems in the micro-gravity environment of space. If strongly attached to the heat transfer surfaces, the water droplets may cause an increase in air pressure drop across the condensing heat exchanger and, consequently, a reduction in performance of the condensing heat exchanger. If not strongly attached, the water drops may become entrapped in the air stream and carried into the cabin. In such instance, rain could quite literally fall inside the cabin.
To address the above water condensation concerns, heat exchanger surfaces have usually been coated with a hydrophilic coating to facilitate wetting and wicking, rather than droplet formation. However, since the hydrophilic coating becomes wet during the operation of the condensing heat exchanger, the moisture provides a potential breeding ground for microbes such as bacteria and fungi. Moisture levels between about 15% and 35% will predominantly support the growth of fungi, while moisture levels above about 35% will also predominantly support the growth of bacteria. The microbial growths could block air passages inside the condensing heat exchanger and, consequently, reduce performance of the condensing heat exchanger. The bacteria and fungi, if allowed to proliferate, could also affect the health of crew members inside the cabin.
In addition to hydrophilic and anti-microbial characteristics, coatings on heat exchangers are often required to be corrosion resistant. This is especially important for aluminum heat exchangers that have a high tendency to corrode. Nevertheless, aluminum is still commonly used because it has high heat conductance and is light in weight.
The hydrophilic coatings in the past have typically been formed from a slurry and then sprayed, dipped or painted onto the heat exchanger surface. As an example, in U.S. Pat. Nos. 5,264,250 and 5,305,827, a slurry is made from an inorganic compound (i.e., hydrophilic material), binder and insolubilizer. Optionally, an anti-microbial agent, such as silver oxide, can be added to the slurry during its formation in order to minimize microbe formation. The inorganic compound can include silica and calcium silicate. Prior to the components in the slurry aggregating, the slurry is dipped, sprayed, painted or flowed onto the heat exchanger surface. Thereafter, the slurry coating is dried and subjected to high temperature sintering at about 500° C. WO No. 95/25143 is related to the above. These coatings, however, do not appear to also be corrosion resistant.
Other hydrophilic coatings have been formed by successive solutions that are dipped, sprayed, or painted onto the heat exchanger surface. In U.S. Pat. No. 5,234,714, an aluminum heat exchanger is successively immersed in a cleaning solution, chromate solution, and then silicate solution. The chromium solution includes an activator with chromium trioxide, hydrofluoric acid, and nitric acid. The silicate solution includes silicate of soda and potassium hydroxide. After the solution baths, the heat exchanger is baked at about 415° F. By maintaining precise concentration levels in the solutions, a corrosive resistant, hydrophilic coating is claimed to be produced. While corrosion resistant, no mechanism is described to make the coating anti-microbial.
Another example of successive solution applications is U.S. Pat. No. 3,989,550. An aluminum surface is first etched and then treated with a first solution of active fluoride, chromate ions, and phosphate ions. A second solution includes an alkali metal silicate composed of silicon dioxide and an alkali metal oxide. The latter is preferably sodium or potassium oxide. After the two solution applications, the coated surface is baked preferably at about 350° F. This coating appears to be corrosive resistant but not anti-microbial.
Due to the belief that coatings containing silicates cause excessive wear on tooling due to ion exchange resin particles or boehmite, EP No. 88303545.3 teaches an aluminum surface having a hydrophilic coating comprised of slurry of activated alumina and an organic binder resin. The resin can be an acrylic, polyester or epoxy. Optionally, the slurry coating can be blended with dispersion stabilizers, catalysts, plasticizers, and cross-linking agents. As with the above techniques, the slurry coating in EP No. 88303545.3 can be applied by dipping, spraying, or brushing individual components prior to assembly. However, such pre-coating methodology tends to have a negative impact on process efficiency since components are first coated and then assembled, as opposed to coating an assembled apparatus.
Additional examples of hydrophilic coatings on heat exchanger surfaces are found in U.S. Pat. No. 5,545,438; EP Nos. 676,250 and 623,653; WO Nos. 96/21752 and 95/10642; and UK Nos. 2,295,828; 2,288,178; and 2,259,514.
Notwithstanding the advantages provided in the above prior art, disadvantages still remain. In terms of making and applying the hydrophilic coating, it can be seen that multiple steps and multiple components (i.e., hydrophilic material, binder, plasticizer, etc.) are required. Yet, there is little synergetic effect. Consequently, compromises have to be made, resulting in non-optimal properties with respect to adhesion, heat conductivity, and service life. Also, the formation of a slurry and/or the use of a pre-assembly dip-coating type technique (i.e., dipping, spraying, painting, brushing) restricts the scope of application. In other words, the ability to coat heat exchanger substrates with small fin sizes and/or complex configurations becomes restricted. For example, the smaller the fin size, the more difficult it is for a slurry to provide a thin coating. Likewise, the more complex the configuration, the more difficult it is to spray or paint the substrate.
Another significant disadvantage in the prior art relates to how the anti-microbial characteristic is employed. For example, even though silver oxide is typically employed, it is intrinsically hydrophobic. When the concentration of the silver oxide exceeds about 1.5% of the total weight of the hydrophilic coating, it is believed that the hydrophilicity is reduced. Thus, increasing the amount of silver oxide increases the likelihood of droplets forming inside the air passageways of the heat exchanger. On the other hand, decreasing the amount of anti-microbial agent is thought to allow the agent to dissolve out of the coating too quickly, thereby causing pitting in the coating. In turn, hydrophilicity, structural integrity, and heat transfer efficiency are reduced. Furthermore, after the anti-microbial agent has completely dissolved, the anti-microbial characteristic cannot be regenerated in the absence of re-coating. Yet another limitation is that only a limited amount of an anti-microbial agent can be used without compromising the processibility of the slurry, the hydrophilicity, and heat transfer properties. An example of an anti-microbial and hydrophilic coating is found in U.S. patent application Ser. No. 60/041,129, which is also assigned to the assignee herein.
As can be seen, there is a need for an improved hydrophilic coating and method of making the same. In particular, an improved hydrophilic coating is needed that is also corrosion and/or microbial resistant. A hydrophilic coating is needed that can be used on heat exchanger substrates that are small in fin size and/or of a complex shape. An ability to regenerate the anti-microbial effects in a hydrophilic coating is also needed. A method of making an improved hydrophilic coating is needed to eliminate the need for multiple types of components in the coating which provide little synergetic effect. Also needed is an improved method that can eliminate the need for multiple steps to produce the coating.